The field of the invention relates generally to combustion furnaces, and more particularly, to a method and apparatus to reduce emissions of trace elements and particulate matter from a combustion furnace flue gas.
During a typical combustion process within a furnace or boiler, for example, a flow of combustion exhaust gas, or flue gas, is produced. Known combustion exhaust gases contain combustion products including, but not limited to, carbon, fly ash, carbon dioxide, carbon monoxide, water, hydrogen, nitrogen, sulfur, chlorine, and/or trace metals, for example, mercury, generated as a result of combusting solid and/or liquid fossil fuels.
Volatile metal mercury is one air pollutant produced through coal combustion. Mercury released from coal during combustion is readily aerosolized and can become airborne. Airborne mercury may travel globally prior to being deposited onto soil and water. Mercury released in the environment is a persistent and toxic pollutant that may accumulate in the food chain. For example, mercury can be transformed within microorganisms into methylmercury. Consumption of contaminated fish is the major route of human exposure to methylmercury.
Mercury emissions from coal-fired power plants are the subject of governmental regulation. The control of mercury emissions is complicated by the several forms mercury may take within combustion flue gas. For example, at combustion temperatures, mercury is present in flue gas in its elemental form, Hg0, which may be difficult to control because elemental mercury is mostly non-reactive and easily volatized. Mercury reacts with halogens, predominately chlorine, present in coal and released into flue gas during combustion as flue gas cools below 1000° F. Such reactions may convert mercury to its highly reactive, oxidized form, Hg+2. Mercury may also be absorbed in fly ash and/or other particulate matter present in the flue gas to form particulate-bound mercury.
Because mercury can take several forms, known control technologies do not effectively control mercury emission for all coal types and for all combustion configurations. Some known mercury control technologies take advantage of mercury's reactivity with carbon and use carbon as a mercury sorbent to remove mercury from flue gas. Carbon may be formed in-situ during the combustion process as a result of incomplete coal combustion or may be injected into mercury-containing flue gas, usually in the form of activated carbon. Further, carbon in the presence of chlorine may increase the oxidation of elemental mercury. In the flue gas, mercury can be converted to its oxidized form, Hg+2, and react with chlorine-containing species to form mercury chloride (HgCl2). As such, the extent of mercury oxidation in flue gas is generally higher for coals with a higher chlorine content, such as bituminous coals, and lower for coals with a lower chlorine content, such as low-rank coals.
Particulate matter is another major pollutant produced by fossil fuel combustion. Various pollutant control techniques when implemented primarily for removal of nitrogen oxides (NOx), e.g., low-NOx burners (LNB) and combustion modifications such as air and fuel staging could lead to increased amounts of particulate matter in the exhaust gas at the furnace exit. Higher particulate matter loadings at the furnace exit increase loads placed on particulate control devices such as electrostatic precipitators and baghouses, and can lead to increased particulate matter emissions into atmosphere. Sometimes combustion modifications are implemented in such a way as to intentionally increase the amount of carbon in fly ash. While unburned carbon (also loosely referred to as loss-on-ignition, or LOI) can serve as effective capturing agent for gaseous pollutants such as mercury, increased levels of LOI in fly ash negatively affect fly ash properties and can lead to additional difficulties associated with fly ash collection, use, and disposal.
Combustion fly ash can be used as a cement additive. Stringent requirements exist both in Europe and North America for the maximum allowed carbon content in fly ash sold as a cement additive. The limitations are typically based on the foaming index of the cement. Carbon distribution in fly ash as a function of ash particle size is usually not uniform. Depending on the grinding characteristics of the coal fuel, unburned carbon content can be higher in the larger ash particles (50 to 200 microns or larger), because these particles are generated as a result of incomplete combustion of the largest coal particles. Conversely, typical size of activated carbon sorbent particles is about 3 to 20 microns because it is advantageous to increase particle surface area and improve gas-solid contact for higher removal efficiency. Therefore, depending on plant configuration and operation, high carbon fractions can be presented mostly in larger fly ash particles, or smaller particles, or both. Most often, the highest amounts of captured mercury are associated with particles having high carbon content. There is growing concern about re-use of mercury-laden fly ash, because captured mercury can later escape.
Efficiencies of most available mercury emission control technologies depend on the mercury speciation in flue gas. Oxidized mercury is water-soluble and may be removed from flue gas using known wet desulfurization systems (wet scrubbers). At least some particulate-bound mercury may be removed from flue gas using known particulate collection systems. Elemental mercury is more difficult to remove than oxidized mercury and/or particulate-bound mercury because elemental mercury is unreactive and, as such, cannot be removed from flue gas with wet desulfurization systems or particulate collection system.
In some known systems, because the concentration of mercury in the flue gas is very small (typically less than 10 parts per billion or ppb), diffusion of mercury from the surrounding flue gases may limit the mercury removal process. Most of the flue gases produced in known systems flow in substantially laminar flow patterns and are characterized by slow diffusion rates. Because of the flow characteristics of the flue gas, some known mercury emission reduction systems have attempted to optimize the use of the sorbent by modifying the number and design of sorbent injection lances to achieve sorbent coverage within the flue duct.